In an internal combustion engine mounted on a vehicle or the like, an exhaust manifold assembly is provided for the discharge of exhaust gas. The exhaust manifold assembly has branch pipes of a number matching the number of cylinders in the engine, and a junction where the branch pipes are joined. Branched exhaust passages formed in the branch pipes are respectively in communication with exhaust ports.
In Japanese Patent Laid-Open No. 4-269322 an exhaust manifold assembly in an internal combustion engine is disclosed. According to this exhaust manifold assembly, in an internal combustion engine having a plurality of cylinders, one junction point is used for junction of branch pipes corresponding to the cylinders to a junction pipe, and each of the branch pipes are arranged so as to intersect the junction pipe at a specified angle, thereby improving the output without exhaust interference.
In Japanese Utility Model Publication No. 7-24576 a catalytic converter in an exhaust manifold is disclosed. According to this catalytic converter, a metal carrier with a catalyst carried therein is inserted and fixed into a cylinder shell. A first case for connection to the exhaust manifold side is fitted in an upstream portion of the cylindrical shell, and a downstream end of the first case and the shell are bonded together by full-circle welding, while a second case for connection to an exhaust pipe is fitted in a downstream portion of the shell and an upstream end of the second case and the shell are bonded together by full-circle welding. The fitting allowance or clearance between the first case and the shell is set larger than the fitting allowance between the second case and the shell.
Reference is further made to Japanese Utility Model Laid-Open No. 64-51719, which also discloses an exhaust manifold structure in an engine. This exhaust manifold structure comprises a first branch passage group having a junction on a downstream side and connected to a first group of cylinders having non-continuous intake strokes, a second branch passage group having a junction on a downstream side and connected to a second group of cylinders having non-continuous intake strokes, and a final junction contiguous to both the junction of the first branch passage group and the junction of the second passage group. The first branch passage group is disposed in front of an engine body, the second branch passage group is disposed between the first branch passage group and the engine body, and an expanded portion which constitutes an expansion chamber is formed over the area from the second branch passage group to the final junction.
In Japanese Utility Model Laid-Open No. 2-103121 an exhaust manifold structure for a multi-cylinder engine is disclosed which has a plurality of exhaust passages connected cylinder by cylinder to the body side of the engine and also has junctions formed at downstream ends of the exhaust passages to join the downstream sides of the exhaust passages into a smaller number of exhaust passages than the number of cylinders. This exhaust manifold structure is of the type in which exhaust passages of a group of exhaust passages overlap one another in a mutually intersecting manner. One of the overlapped exhaust passages has a rib extending substantially along the axis of the other exhaust passage. According to a similar type of an exhaust manifold structure for an engine, a rib is formed between two substantially parallel exhaust passages so as to extend up to an outside position beyond the center of at least one of the two exhaust passages.
Further, in Japanese Utility Model Laid-Open No. 5-14524 an exhaust manifold assembly is disclosed in which a joined position of cylinder exhaust pipes in a multi-cylinder four-cycle engine is opened to form an empty chamber.
Heretofore, exhaust manifold assemblies of various structures have been developed in an effort to prevent a decrease of engine torque and output caused by exhaust interference from the internal combustion engine.
For example, as shown in FIG. 16., a known exhaust manifold 102 is formed as a one-point junction type by first to fourth branch pipes 104-1, 104-2, 104-3 and 104-4, as independent ports, and a junction 132 where those four branch pipes are joined. The first to fourth branch pipes 104-1, 104-2, 104-3 and 104-4 are formed long so as to be of substantially the same length.
However, the structure for the elimination of exhaust interference requires the four branch pipes to be long and thus layout restrictions are great, giving rise to the inconvenience that it is impossible to make an effective utilization of space.
In the known exhaust manifold structure shown in FIG. 17, an exhaust manifold 202 comprises first to fourth branch pipes 204-1, 204-2, 204-3 and 204-4 and a junction 232 where those four branch pipes are joined. In the same structure, non-interfering ports, namely, two ports which are not continuous (that is, the cylinders connected to these two ports are not directly fired in sequence one after the other), are joined in accordance with ignition order. More particularly, the first and fourth branch pipes 204-1, 204-4 are joined, and the second and third branch pipes 204-2, 204-3 are joined, followed by being combined together in the junction 232.
As a result, the structure of the junction and that of the vicinity thereof becomes somewhat more compact, but this arrangement is disadvantageous in practical use because it still occupies a large space.
Further, in the known exhaust manifold structure shown in FIG. 18, a single stainless steel pipe is bent in an L-shape to form a first branch pipe 304-1 and a main pipe 334, and second to fourth branch pipes 304-2, 304-3 and 304-4 are connected to the main pipe 334 to form a ladder-shaped (or ladder type) exhaust manifold 302. The first branch pipe 304-1 is larger in diameter than the second to fourth branch pipes 304-2, 304-3 and 304-4. The reference numeral 306 denotes a cylinder head-side mounting flange portion formed by a steel plate, numeral 308 denotes an exhaust pipe-side mounting flange portion formed by a steel plate, numeral 312 denotes an EGR (exhaust gas recirculation) pipe, numeral 314 denotes a stud bolt, numeral 316 denotes a catalyst case disposed just under the exhaust manifold 302, and numeral 318 denotes an exhaust pipe.
However, in the manifold converter type with the catalyst case disposed just under the exhaust manifold, as shown in FIG. 18, exhaust interference cannot be eliminated due to layout restrictions.
As to an exhaust manifold cover which is mounted for shutting out heat and sound from the exhaust manifold, as shown in FIGS. 19 and 20, an exhaust manifold cover 122 is in many cases formed by a plurality of joined planar portions. This shape is inferior in both strength and appearance as compared with a streamlined shape.
Further, in the exhaust manifold cover 122 there is virtually no planar portion which extends in the horizontal direction, and hence there is no space for the placement of information such as instructions or an operating method. This is disadvantageous in practical use.